I. Field of the Invention
This invention relates generally to millimeter wave RF power sources for use in transmitting systems, and more particularly to a Ka-band microstrip, high-power RF supply employing a plurality of voltage controlled oscillators whose outputs are power combined in such a way that the plural oscillators become frequency locked by the mutual injection on a signal component from one oscillator into the other through the power combining network.
II. Discussion of the Prior Art
A voltage controlled oscillator (VCO) is an electronic circuit which produces an output signal whose frequency is determined by the amplitude of an externally applied tuning signal. Such oscillators are fundamental components in many radio frequency (RD) systems. For example, they may be found in millimeter wave communication systems, in transmitters for radar and in the seekers or sensors used in extended range precision weapon systems. However, it is found that many applications will require more RF power than can be furnished by a single VCO, especially when working in the MMW frequencies. Thus, various prior art power combining techniques have been devised. At MMW frequencies, wave guide combiners are typically used. In this regard, reference is made to the McClymonds U.S. Pat. No. 4,583,058 as well as to the Sigmon et al U.S. Pat. No. 4,571,555 and the Crapuchettes U.S. Pat. No. 2,852,678. Because of their relatively large size, weight and expense, such an approach may not be an alternative in many applications.
Power combined oscillators of the prior art are generally designed to work at a single, fixed, operating frequency, rather than being variable frequency oscillators. Again, this oft times makes them unsuitable for certain applications. Over and above that drawback, the individual, fixed frequency oscillators used in such prior art circuits must be carefully matched and require exact tuning to assure that each oscillator will be operating at the same frequency. The requirement for careful device matching and exact tuning necessarily increases the cost of such systems quite significantly.
The Jacobs et al U.S. Pat. No. 4,544,680 discloses a triple Gunn diode, self-oscillating, mixer system designed to operate at millimeter wave frequencies. In this system, two fixed frequency Gunn diode cavity oscillators are arranged to mutually injection lock one another by way of a coxial cable connection extending between them. This circuit is designed to assure that the phase difference between the two mutual injection lock signals is 180.degree.. The device of the Jacobs patent is not strictly a power combiner, but is actually a mixer. The signal outputs from the two oscillators in the Jacobs system are applied to an image line 180.degree. hybrid and if the two signals are exactly 180.degree. out-of-phase with respect to one another and are of generally equal amplitude, the algebraic sum of the two signals will appear at a single output port. It is to be noted that, in the Jacobs system, fixed frequency Gunn diode cavity oscillators are employed rather than microstrip implemented VCO's. Furthermore, rather than employing a power combining network whose isolation capability is less than total, in the Jacobs system, a coaxial cable is employed to mutually injection lock the two oscillators and power combining efficiency depends significantly upon the near-ideal isolation characteristics of the combining network.
Fixed frequency power combined oscillators can be made to operate as a variable frequency system by using an external source and an isolating/injecting network, such as a circulator. This technique of changing the frequency of fixed frequency oscillators by injection locking them to an external variable oscillator is well known to those versed in the art. The disadvantages of this approach are the need for an external variable frequency oscillator and a high performance (low loss/high isolation) circulator. This increases the cost, size and complexity of the power combined oscillator system.